test.php

<?php
class DataObject
{
  public $a;
  public $b;

  public function __toString()
  {
    return (string)$this->a . ','. $this->b;
  }
}

function test($object) {
  $object->b = "c";
}


$object = new DataObject();
$object->a = "a";
$object->b = "b";

test($object);

echo $object;